Mass flow controllers have been widely used as flow rate controllers in gas supply systems in semiconductor manufacturing facilities. In recent years, pressure-type flow controllers have been developed to replace the mass flow controllers.
FIG. 20 shows the configuration of the pressure-type flow controller, which the inventors developed earlier and disclosed in unexamined Japanese patent publication No. 08-338546. The controller is based on the principle of calculating the fluid flow rate Q on the downstream side of an orifice 5 with the equation Q=KP1 (K=a constant) with the ratio P2/P1 of pressure P2 on the downstream side of the orifice 5 to the upstream pressure P1 kept below the ratio of the gas critical pressure.
Referring to FIG. 20, reference numeral 1 indicates a pressure-type flow rate controller; 2, a control valve; 3, a valve drive; 4, a pressure detector; 5, an orifice; 7, a control unit; 7a, a temperature correction circuit; 7b, a flow rate calculation circuit; 7c, a comparison circuit; 7d, an amplification circuit; 21a and 21b, amplification circuits; 22a and 22b, analog to digital (A/D) conversion circuits; 24, an inversion amplifier; 25, a valve; Qy, a control signal; Qc, a calculation signal; and Os, a set flow rate signal.
The pressure type flow rate controller illustrated in FIG. 20 permits control with high precision of the flow Q on the downstream side of the orifice by regulating the pressure P1 on the upstream side through actuation of control valve 2. The flow rate controller has proved to be very useful in practice.
However, this pressure type flow control system still has many problems yet to be solved. Foremost among them are those concerned with the sonic velocity nozzle (orifice).
The first problem is the cost for manufacturing the orifice. The flow control system sometimes requires orifices with a bore diameter of 10 .mu.m to 0.8 mm .O slashed.. Sonic velocity orifices with bore diameters in the range 10 .mu.m to 0.8 mm .O slashed. are usually machined by electric discharge or etching. Machining the orifice by electric discharge or etching to a specific configuration, for example, the configuration specified in ISO 9300 boosts the manufacturing costs too much.
The second problem is uniformity in orifice flow characteristics. Sonic orifices with a fine bore are difficult to machine uniformly, such that it is difficult for the flow rate characteristics of different orifices to uniformly fall in a specific range and the flow rate characteristics tend to vary widely from orifice to orifice. The result is low accuracy in measurement of the flow rate, and correction of measurement takes too much time and trouble.
The third problem is a range of linearity of the flow rate characteristics curve. If the ratio P2/P1 of the downstream pressure P2 to the upstream pressure P1 drops below a critical pressure ratio of gas (in the case of air and nitrogen, some 0.5), the flow velocity of the gas passing through the orifice will reach sonic velocity. Then the pressure change on the downstream side of the orifice will no longer propagate to the upstream side. As a result, it is believed that it is possible to obtain a stable mass flow rate corresponding to the state upstream of the orifice.
The problem is that, according to the actual flow rate characteristic curve, the pressure ratio range where the so-called linearity characteristics are applicable, that is, where the flow rate calculation can be made with the equation Q=KP1, is far below the critical gas pressure ratio point. That is, the range where the flow rate is controllable is correspondingly limited and narrowed.
Further, because there exist problems with the linearity characteristic itself, a high degree of linearity is difficult to obtain, and any further improvement in flow control precision cannot be expected from the prior art.